Wireless communication method for uplink multiple-user transmission schedule and wireless communication terminal using the method

ABSTRACT

The present invention relates to a wireless communication method for uplink multi-user transmission scheduling and a wireless communication terminal using the same. To this end, provided are a base wireless communication terminal comprising a processor and a transceiver, wherein the processor transmits a trigger frame soliciting an uplink multi-user transmission, determines whether the uplink multi-user transmission is successful based on received uplink multi-user data in response to the trigger frame, and retransmits the trigger frame when it is determined that the uplink multi-user transmission has failed and a wireless communication method using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/739,161 filed on Jun. 5, 2018, which is the U.S. National Stage ofInternational Patent Application No. PCT/KR2016/006856 filed on Jun. 27,2016, which claims the priority to Korean Patent Application No.10-2015-0091532 filed in the Korean Intellectual Property Office on Jun.26, 2015, Korean Patent Application No. 10-2015-0096770 filed in theKorean Intellectual Property Office on Jul. 7, 2015, Korean PatentApplication No. 10-2015-0101505 filed in the Korean IntellectualProperty Office on Jul. 17, 2015 and Korean Patent Application No.10-2015-0104659 filed in the Korean Intellectual Property Office on Jul.23, 2015, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a wireless communication method foruplink multi-user transmission scheduling and a wireless communicationterminal using the same, and more particularly, to a wirelesscommunication method and a wireless communication terminal forefficiently scheduling simultaneous transmission of a plurality ofterminals.

BACKGROUND ART

In recent years, with supply expansion of mobile apparatuses, a wirelessLAN technology that can provide a rapid wireless Internet service to themobile apparatuses has been significantly spotlighted. The wireless LANtechnology allows mobile apparatuses including a smart phone, a smartpad, a laptop computer, a portable multimedia player, an embeddedapparatus, and the like to wirelessly access the Internet in home or acompany or a specific service providing area based on a wirelesscommunication technology in a short range.

Institute of Electrical and Electronics Engineers (IEEE) 802.11 hascommercialized or developed various technological standards since aninitial wireless LAN technology is supported using frequencies of 2.4GHz. First, the IEEE 802.11b supports a communication speed of a maximumof 11 Mbps while using frequencies of a 2.4 GHz band. IEEE 802.11a whichis commercialized after the IEEE 802.11b uses frequencies of not the 2.4GHz band but a 5 GHz band to reduce an influence by interference ascompared with the frequencies of the 2.4 GHz band which aresignificantly congested and improves the communication speed up to amaximum of 54 Mbps by using an OFDM technology. However, the IEEE802.11a has a disadvantage in that a communication distance is shorterthan the IEEE 802.11b. In addition, IEEE 802.11g uses the frequencies ofthe 2.4 GHz band similarly to the IEEE 802.11b to implement thecommunication speed of a maximum of 54 Mbps and satisfies backwardcompatibility to significantly come into the spotlight and further, issuperior to the IEEE 802.11a in terms of the communication distance.

Moreover, as a technology standard established to overcome a limitationof the communication speed which is pointed out as a weak point in awireless LAN, IEEE 802.11n has been provided. The IEEE 802.11n aims atincreasing the speed and reliability of a network and extending anoperating distance of a wireless network. In more detail, the IEEE802.11n supports a high throughput (HT) in which a data processing speedis a maximum of 540 Mbps or more and further, is based on a multipleinputs and multiple outputs (MIMO) technology in which multiple antennasare used at both sides of a transmitting unit and a receiving unit inorder to minimize a transmission error and optimize a data speed.Further, the standard can use a coding scheme that transmits multiplecopies which overlap with each other in order to increase datareliability.

As the supply of the wireless LAN is activated and further, applicationsusing the wireless LAN are diversified, the need for new wireless LANsystems for supporting a higher throughput (very high throughput (VHT))than the data processing speed supported by the IEEE 802.11n has comeinto the spotlight. Among them, IEEE 802.11ac supports a wide bandwidth(80 to 160 MHz) in the 5 GHz frequencies. The IEEE 802.11ac standard isdefined only in the 5 GHz band, but initial 11ac chipsets will supporteven operations in the 2.4 GHz band for the backward compatibility withthe existing 2.4 GHz band products. Theoretically, according to thestandard, wireless LAN speeds of multiple stations are enabled up to aminimum of 1 Gbps and a maximum single link speed is enabled up to aminimum of 500 Mbps. This is achieved by extending concepts of awireless interface accepted by 802.11n, such as a wider wirelessfrequency bandwidth (a maximum of 160 MHz), more MIMO spatial streams (amaximum of 8), multi-user MIMO, and high-density modulation (a maximumof 256 QAM). Further, as a scheme that transmits data by using a 60 GHzband instead of the existing 2.4 GHz/5 GHz, IEEE 802.11ad has beenprovided. The IEEE 802.11ad is a transmission standard that provides aspeed of a maximum of 7 Gbps by using a beamforming technology and issuitable for high bit rate moving picture streaming such as massive dataor non-compression HD video. However, since it is difficult for the 60GHz frequency band to pass through an obstacle, it is disadvantageous inthat the 60 GHz frequency band can be used only among devices in ashort-distance space.

Meanwhile, in recent years, as next-generation wireless LAN standardsafter the 802.11ac and 802.11ad, discussion for providing ahigh-efficiency and high-performance wireless LAN communicationtechnology in a high-density environment is continuously performed. Thatis, in a next-generation wireless LAN environment, communication havinghigh frequency efficiency needs to be provided indoors/outdoors underthe presence of high-density stations and access points (APs) andvarious technologies for implementing the communication are required.

DISCLOSURE Technical Problem

The present invention has an object to providehigh-efficiency/high-performance wireless LAN communication in ahigh-density environment as described above.

The present invention has an object to perform efficient scheduling ofdata transmission of each terminal in a situation where a downlinksingle-user transmission, a downlink multi-user transmission, an uplinksingle-user transmission and uplink multi-user transmission are mixed.

In addition, the present invention has an object to define a subsequentoperation method when there is no response to a trigger frame for uplinkmulti-user transmission.

Technical Solution

In order to achieve the objects, the present invention provides awireless communication method and a wireless communication terminal asbelow.

First, an exemplary embodiment of the present invention provides a basewireless communication terminal comprising a processor and atransceiver, wherein the processor transmits a trigger frame solicitingan uplink multi-user transmission, determines whether the uplinkmulti-user transmission is successful based on received uplinkmulti-user data in response to the trigger frame, and retransmits thetrigger frame when it is determined that the uplink multi-usertransmission has failed.

The processor may determine that the uplink multi-user transmission issuccessful when uplink data is received from at least one of wirelesscommunication terminals indicated by the trigger frame.

The processor may determine that the uplink multi-user transmission hasfailed when no uplink data in response to the trigger frame has beenreceived.

When it is determined that the uplink multi-user transmission hasfailed, the processor may obtain a new backoff counter forretransmitting the trigger frame, and perform a backoff procedure basedon the obtained new backoff counter.

The processor may retransmit the trigger frame when the channel is idlefor a predetermined period of time after the trigger frame has beentransmitted.

The predetermined period of time may be set based on a time forreceiving a preamble of an uplink multi-user PLCP protocol data unit(PPPU).

The predetermined period of time may be set to be a PCF IFS (PIFS).

The processor may perform the retransmission of the trigger frame untilan uplink multi-user transmission based on the retransmission issuccessful within a predetermined count limit of retransmission or apredetermined time limit of retransmission.

The time limit of retransmission may be set based on an initialtransmission opportunity (TXOP) set in the trigger frame.

The processor may attempt to transmit a new trigger frame based on newscheduling information when all the uplink multi-user transmissionsbased on the retransmission within the count limit of retransmission orthe time limit of retransmission have failed.

In addition, an exemplary embodiment of the present invention provides awireless communication method of a base wireless communication terminal,including: transmitting a trigger frame soliciting an uplink multi-usertransmission; determining whether the uplink multi-user transmission issuccessful based on received uplink multi-user data in response to thetrigger frame; and retransmitting the trigger frame when it isdetermined that the uplink multi-user transmission has failed.

Another exemplary embodiment of the present invention provides a basewireless communication terminal comprising a processor and atransceiver, wherein the processor transmits a first trigger framesoliciting an uplink multi-user transmission, wherein the trigger frameincludes resource unit allocation information for wireless communicationterminals to perform uplink multi-user transmission, receives uplinkmulti-user data in response to the transmitted trigger frame, generatesnew scheduling information for wireless communication terminals thathave failed to transmit the uplink multi-user data, and transmits asecond trigger frame based on the generated new scheduling information.

When uplink data is not received from a wireless communication terminalto which a first resource unit is allocated through the first triggerframe, the processor may allocate a second resource unit to thecorresponding wireless communication terminal through the second triggerframe.

The first trigger frame and the second trigger frame may be differenttrigger frames transmitted within a predetermined period for consecutiveuplink multi-user transmissions.

The processor may receive a buffer status report from at least onewireless communication terminal, and transmit the trigger frame based onthe received buffer status report, wherein the buffer status report istransmitted based on a probability value according to an access categoryof data to be transmitted by the wireless communication terminal.

The higher a priority of an access category of data to be transmittedis, the higher a transmission probability of the buffer status reportmay be.

In addition, another exemplary embodiment of the present inventionprovides a wireless communication method of a base wirelesscommunication terminal, including: transmitting a first trigger framesoliciting an uplink multi-user transmission, wherein the trigger frameincludes resource unit allocation information for wireless communicationterminals to perform uplink multi-user transmission; receiving uplinkmulti-user data in response to the transmitted trigger frame; generatingnew scheduling information for wireless communication terminals thathave failed to transmit the uplink multi-user data; and transmitting asecond trigger frame based on the generated new scheduling information.

Advantageous Effects

According to the embodiment of the present invention, the reliability ofthe uplink multi-user transmission can be ensured and the performancethereof can be improved through efficient scheduling.

According to another embodiment of the present invention, it is possibleto minimize the resource loss by controlling the consecutive uplinkmulti-user transmission processes depending on the congestion state ofthe network.

According to an embodiment of the present invention, it is possible toincrease the total resource utilization rate in the contention-basedchannel access system and improve the performance of the wireless LANsystem.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless LAN system according to an embodiment ofthe present invention.

FIG. 2 illustrates a wireless LAN system according to another embodimentof the present invention.

FIG. 3 illustrates a configuration of a station according to anembodiment of the present invention.

FIG. 4 illustrates a configuration of an access point according to anembodiment of the present invention.

FIG. 5 schematically illustrates a process in which a STA and an AP seta link.

FIG. 6 illustrates a carrier sense multiple access (CSMA)/collisionavoidance (CA) method used in wireless LAN communication.

FIG. 7 illustrates a method for performing a distributed coordinationfunction (DCF) using a request to send (RTS) frame and a clear to send(CTS) frame.

FIG. 8 illustrates an embodiment in which a trigger frame isretransmitted in an uplink multi-user transmission process.

FIGS. 9 and 10 illustrate a trigger frame retransmission methodaccording to an embodiment of the present invention.

FIGS. 11 and 12 illustrate a scheduling method according to consecutivefailure of an uplink multi-user transmission process.

FIGS. 13 and 14 illustrate an embodiment in which a new trigger frame istransmitted after a failure of the uplink multi-user transmissionprocess.

FIGS. 15 and 16 illustrate a trigger frame retransmission methodaccording to another embodiment of the present invention.

FIGS. 17 and 18 illustrate a trigger frame retransmission methodaccording to yet another embodiment of the present invention.

FIGS. 19 to 21 illustrate consecutive uplink multi-user transmissionmethods according to an embodiment of the present invention.

FIG. 22 illustrates a method of a buffer status report and an uplinkmulti-user transmission based thereon according to an embodiment of thepresent invention.

FIGS. 23 and 24 illustrate an uplink multi-user transmission processaccording to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Terms used in the specification adopt general terms which are currentlywidely used by considering functions in the present invention, but theterms may be changed depending on an intention of those skilled in theart, customs, and emergence of new technology. Further, in a specificcase, there is a term arbitrarily selected by an applicant and in thiscase, a meaning thereof will be described in a corresponding descriptionpart of the invention. Accordingly, it should be revealed that a termused in the specification should be analyzed based on not just a name ofthe term but a substantial meaning of the term and contents throughoutthe specification.

Throughout this specification and the claims that follow, when it isdescribed that an element is “coupled” to another element, the elementmay be “directly coupled” to the other element or “electrically coupled”to the other element through a third element. Further, unless explicitlydescribed to the contrary, the word “comprise” and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof stated elements but not the exclusion of any other elements.Moreover, limitations such as “or more” or “or less” based on a specificthreshold may be appropriately substituted with “more than” or “lessthan”, respectively.

This application claims priority to and the benefit of Korean PatentApplication Nos. 10-2015-0091532, 10-2015-0096770, 10-2015-0101505 and10-2015-0104659 filed in the Korean Intellectual Property Office and theembodiments and mentioned items described in the respective application,which forms the basis of the priority, shall be included in the DetailedDescription of the present application.

FIG. 1 is a diagram illustrating a wireless LAN system according to anembodiment of the present invention. The wireless LAN system includesone or more basic service sets (BSS) and the BSS represents a set ofapparatuses which are successfully synchronized with each other tocommunicate with each other. In general, the BSS may be classified intoan infrastructure BSS and an independent BSS (IBSS) and FIG. 1illustrates the infrastructure BSS between them.

As illustrated in FIG. 1 , the infrastructure BSS (BSS1 and BSS2)includes one or more stations STA1, STA2, STA3, STA4, and STA5, accesspoints PCP/AP-1 and PCP/AP-2 which are stations providing a distributionservice, and a distribution system (DS) connecting the multiple accesspoints PCP/AP-1 and PCP/AP-2.

The station (STA) is a predetermined device including medium accesscontrol (MAC) following a regulation of an IEEE 802.11 standard and aphysical layer interface for a wireless medium, and includes both anon-access point (non-AP) station and an access point (AP) in a broadsense. Further, in the present specification, a term ‘terminal’ may beused to refer to a non-AP STA, or an AP, or to both terms. A station forwireless communication includes a processor and a transceiver andaccording to the embodiment, may further include a user interface unitand a display unit. The processor may generate a frame to be transmittedthrough a wireless network or process a frame received through thewireless network and besides, perform various processing for controllingthe station. In addition, the transceiver is functionally connected withthe processor and transmits and receives frames through the wirelessnetwork for the station.

The access point (AP) is an entity that provides access to thedistribution system (DS) via wireless medium for the station associatedtherewith. In the infrastructure BSS, communication among non-APstations is, in principle, performed via the AP, but when a direct linkis configured, direct communication is enabled even among the non-APstations. Meanwhile, in the present invention, the AP is used as aconcept including a personal BSS coordination point (PCP) and mayinclude concepts including a centralized controller, a base station(BS), a node-B, a base transceiver system (BTS), and a site controllerin a broad sense. In the present invention, an AP may also be referredto as a base wireless communication terminal. The base wirelesscommunication terminal may be used as a term which includes an AP, abase station, an eNB (i.e. eNodeB) and a transmission point (TP) in abroad sense. In addition, the base wireless communication terminal mayinclude various types of wireless communication terminals that allocatemedium resources and perform scheduling in communication with aplurality of wireless communication terminals.

A plurality of infrastructure BSSs may be connected with each otherthrough the distribution system (DS). In this case, a plurality of BSSsconnected through the distribution system is referred to as an extendedservice set (ESS).

FIG. 2 illustrates an independent BSS which is a wireless LAN systemaccording to another embodiment of the present invention. In theembodiment of FIG. 2 , duplicative description of parts, which are thesame as or correspond to the embodiment of FIG. 1 , will be omitted.

Since a BSS3 illustrated in FIG. 2 is the independent BSS and does notinclude the AP, all stations STA6 and STA7 are not connected with theAP. The independent BSS is not permitted to access the distributionsystem and forms a self-contained network. In the independent BSS, therespective stations STA6 and STA7 may be directly connected with eachother.

FIG. 3 is a block diagram illustrating a configuration of a station 100according to an embodiment of the present invention.

As illustrated in FIG. 3 , the station 100 according to the embodimentof the present invention may include a processor 110, a transceiver 120,a user interface unit 140, a display unit 150, and a memory 160.

First, the transceiver 120 transmits and receives a wireless signal suchas a wireless LAN packet, or the like and may be embedded in the station100 or provided as an exterior. According to the embodiment, thetransceiver 120 may include at least one transmit/receive module usingdifferent frequency bands. For example, the transceiver 120 may includetransmit/receive modules having different frequency bands such as 2.4GHz, 5 GHz, and 60 GHz. According to an embodiment, the station 100 mayinclude a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the AP or an external station according to a wirelessLAN standard of a frequency band supported by the correspondingtransmit/receive module. The transceiver 120 may operate only onetransmit/receive module at a time or simultaneously operate multipletransmit/receive modules together according to the performance andrequirements of the station 100. When the station 100 includes aplurality of transmit/receive modules, each transmit/receive module maybe implemented by independent elements or a plurality of modules may beintegrated into one chip. In an embodiment of the present invention, thetransceiver 120 may represent a radio frequency (RF) transceiver modulefor processing an RF signal.

Next, the user interface unit 140 includes various types of input/outputmeans provided in the station 100. That is, the user interface unit 140may receive a user input by using various input means and the processor110 may control the station 100 based on the received user input.Further, the user interface unit 140 may perform output based on acommand of the processor 110 by using various output means.

Next, the display unit 150 outputs an image on a display screen. Thedisplay unit 150 may output various display objects such as contentsexecuted by the processor 110 or a user interface based on a controlcommand of the processor 110, and the like. Further, the memory 160stores a control program used in the station 100 and various resultingdata. The control program may include an access program required for thestation 100 to access the AP or the external station.

The processor 110 of the present invention may execute various commandsor programs and process data in the station 100. Further, the processor110 may control the respective units of the station 100 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 110 may execute the program foraccessing the AP stored in the memory 160 and receive a communicationconfiguration message transmitted by the AP. Further, the processor 110may read information on a priority condition of the station 100 includedin the communication configuration message and request the access to theAP based on the information on the priority condition of the station100. The processor 110 of the present invention may represent a maincontrol unit of the station 100 and according to the embodiment, theprocessor 110 may represent a control unit for individually controllingsome component of the station 100, for example, the transceiver 120, andthe like. That is, the processor 110 may be a modem or amodulator/demodulator for modulating and demodulating wireless signalstransmitted to and received from the transceiver 120. The processor 110controls various operations of wireless signal transmission/reception ofthe station 100 according to the embodiment of the present invention. Adetailed embodiment thereof will be described below.

The station 100 illustrated in FIG. 3 is a block diagram according to anembodiment of the present invention, where separate blocks areillustrated as logically distinguished elements of the device.Accordingly, the elements of the device may be mounted in a single chipor multiple chips depending on design of the device. For example, theprocessor 110 and the transceiver 120 may be implemented while beingintegrated into a single chip or implemented as a separate chip.Further, in the embodiment of the present invention, some components ofthe station 100, for example, the user interface unit 140 and thedisplay unit 150 may be optionally provided in the station 100.

FIG. 4 is a block diagram illustrating a configuration of an AP 200according to an embodiment of the present invention.

As illustrated in FIG. 4 , the AP 200 according to the embodiment of thepresent invention may include a processor 210, a transceiver 220, and amemory 260. In FIG. 4 , among the components of the AP 200, duplicativedescription of parts which are the same as or correspond to thecomponents of the station 100 of FIG. 2 will be omitted.

Referring to FIG. 4 , the AP 200 according to the present inventionincludes the transceiver 220 for operating the BSS in at least onefrequency band. As described in the embodiment of FIG. 3 , thetransceiver 220 of the AP 200 may also include a plurality oftransmit/receive modules using different frequency bands. That is, theAP 200 according to the embodiment of the present invention may includetwo or more transmit/receive modules among different frequency bands,for example, 2.4 GHz, 5 GHz, and 60 GHz together. Preferably, the AP 200may include a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the station according to a wireless LAN standard of afrequency band supported by the corresponding transmit/receive module.The transceiver 220 may operate only one transmit/receive module at atime or simultaneously operate multiple transmit/receive modulestogether according to the performance and requirements of the AP 200. Inan embodiment of the present invention, the transceiver 220 mayrepresent a radio frequency (RF) transceiver module for processing an RFsignal.

Next, the memory 260 stores a control program used in the AP 200 andvarious resulting data. The control program may include an accessprogram for managing the access of the station. Further, the processor210 may control the respective units of the AP 200 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 210 may execute the program foraccessing the station stored in the memory 260 and transmitcommunication configuration messages for one or more stations. In thiscase, the communication configuration messages may include informationabout access priority conditions of the respective stations. Further,the processor 210 performs an access configuration according to anaccess request of the station. According to an embodiment, the processor210 may be a modem or a modulator/demodulator for modulating anddemodulating wireless signals transmitted to and received from thetransceiver 220. The processor 210 controls various operations such aswireless signal transmission/reception of the AP 200 according to theembodiment of the present invention. A detailed embodiment thereof willbe described below.

FIG. 5 is a diagram schematically illustrating a process in which a STAsets a link with an AP.

Referring to FIG. 5 , the link between the STA 100 and the AP 200 is setthrough three steps of scanning, authentication, and association in abroad way. First, the scanning step is a step in which the STA 100obtains access information of BSS operated by the AP 200. A method forperforming the scanning includes a passive scanning method in which theAP 200 obtains information by using a beacon message (S101) which isperiodically transmitted and an active scanning method in which the STA100 transmits a probe request to the AP (S103) and obtains accessinformation by receiving a probe response from the AP (S105).

The STA 100 that successfully receives wireless access information inthe scanning step performs the authentication step by transmitting anauthentication request (S107 a) and receiving an authentication responsefrom the AP 200 (S107 b). After the authentication step is performed,the STA 100 performs the association step by transmitting an associationrequest (S109 a) and receiving an association response from the AP 200(S109 b). In this specification, an association basically means awireless association, but the present invention is not limited thereto,and the association may include both the wireless association and awired association in a broad sense.

Meanwhile, an 802.1X based authentication step (S111) and an IP addressobtaining step (S113) through DHCP may be additionally performed. InFIG. 5 , the authentication server 300 is a server that processes 802.1Xbased authentication with the STA 100 and may be present in physicalassociation with the AP 200 or present as a separate server.

FIG. 6 is a diagram illustrating a carrier sense multiple access(CSMA)/collision avoidance (CA) method used in wireless LANcommunication.

A terminal that performs a wireless LAN communication checks whether achannel is busy by performing carrier sensing before transmitting data.When a wireless signal having a predetermined strength or more issensed, it is determined that the corresponding channel is busy and theterminal delays the access to the corresponding channel. Such a processis referred to as clear channel assessment (CCA) and a level to decidewhether the corresponding signal is sensed is referred to as a CCAthreshold. When a wireless signal having the CCA threshold or more,which is received by the terminal, indicates the corresponding terminalas a receiver, the terminal processes the received wireless signal.Meanwhile, when a wireless signal is not sensed in the correspondingchannel or a wireless signal having a strength smaller than the CCAthreshold is sensed, it is determined that the channel is idle.

When it is determined that the channel is idle, each terminal havingdata to be transmitted performs a backoff procedure after an interframespace (IFS) time depending on a situation of each terminal, forinstance, an arbitration IFS (AIFS), a PCF IFS (PIFS), or the likeelapses. According to the embodiment, the AIFS may be used as acomponent which substitutes for the existing DCF IFS (DIFS). Eachterminal stands by while decreasing slot time(s) as long as a randomnumber assigned to the corresponding terminal during an interval of anidle state of the channel and a terminal that completely exhausts theslot time(s) attempts to access the corresponding channel. As such, aninterval in which each terminal performs the backoff procedure isreferred to as a contention window interval.

When a specific terminal successfully accesses the channel, thecorresponding terminal may transmit data through the channel. However,when the terminal which attempts the access collides with anotherterminal, the terminals which collide with each other are assigned withnew random numbers, respectively to perform the backoff procedure again.According to an embodiment, a random number newly assigned to eachterminal may be decided within a range (2*CW) which is twice larger thana range (a contention window, CW) of a random number which thecorresponding terminal is previously assigned. Meanwhile, each terminalattempts the access by performing the backoff procedure again in a nextcontention window interval and in this case, each terminal performs thebackoff procedure from slot time(s) which remained in the previouscontention window interval. By such a method, the respective terminalsthat perform the wireless LAN communication may avoid a mutual collisionfor a specific channel.

FIG. 7 is a diagram illustrating a method for performing a distributedcoordination function using a request to send (RTS) frame and a clear tosend (CTS) frame.

The AP and STAs in the BSS contend in order to obtain an authority fortransmitting data. When data transmission at the previous step iscompleted, each terminal having data to be transmitted performs abackoff procedure while decreasing a backoff counter (alternatively, abackoff timer) of a random number assigned to each terminal after anAFIS time. A transmitting terminal in which the backoff counter expirestransmits the request to send (RTS) frame to notify that correspondingterminal has data to transmit. According to an exemplary embodiment ofFIG. 7 , STA1 which holds a lead in contention with minimum backoff maytransmit the RTS frame after the backoff counter expires. The RTS frameincludes information on a receiver address, a transmitter address, andduration. A receiving terminal (i.e., the AP in FIG. 7 ) that receivesthe RTS frame transmits the clear to send (CTS) frame after waiting fora short IFS (SIFS) time to notify that the data transmission isavailable to the transmitting terminal STA1. The CTS frame includes theinformation on a receiver address and duration. In this case, thereceiver address of the CTS frame may be set identically to atransmitter address of the RTS frame corresponding thereto, that is, anaddress of the transmitting terminal STA1.

The transmitting terminal STA1 that receives the CTS frame transmits thedata after a SIFS time. When the data transmission is completed, thereceiving terminal AP transmits an acknowledgment (ACK) frame after aSIFS time to notify that the data transmission is completed. When thetransmitting terminal receives the ACK frame within a predeterminedtime, the transmitting terminal regards that the data transmission issuccessful. However, when the transmitting terminal does not receive theACK frame within the predetermined time, the transmitting terminalregards that the data transmission is failed. Meanwhile, adjacentterminals that receive at least one of the RTS frame and the CTS framein the course of the transmission procedure set a network allocationvector (NAV) and do not perform data transmission until the set NAV isterminated. In this case, the NAV of each terminal may be set based on aduration field of the received RTS frame or CTS frame.

In the course of the aforementioned data transmission procedure, whenthe RTS frame or CTS frame of the terminals is not normally transferredto a target terminal (i.e., a terminal of the receiver address) due to asituation such as interference or a collision, a subsequent process issuspended. The transmitting terminal STA1 that transmitted the RTS frameregards that the data transmission is unavailable and participates in anext contention by being assigned with a new random number. In thiscase, the newly assigned random number may be determined within a range(2*CW) twice larger than a previous predetermined random number range (acontention window, CW).

<Uplink Multi-User Transmission>

When using orthogonal frequency division multiple access (OFDMA) ormulti-input multi-output (MIMO), one wireless communication terminal cansimultaneously transmit data to a plurality of wireless communicationterminals. Further, one wireless communication terminal cansimultaneously receive data from a plurality of wireless communicationterminals. For example, a downlink multi-user (DL-MU) transmission inwhich an AP simultaneously transmits data to a plurality of STAs, and anuplink multi-user (UL-MU) transmission in which a plurality of STAssimultaneously transmit data to the AP may be performed.

In order to perform the UL-MU transmission, the channel to be used andthe transmission start time of each STA that performs uplinktransmission should be adjusted. In order to efficiently schedule theUL-MU transmission, state information of each STA needs to betransmitted to the AP. According to an embodiment of the presentinvention, information for scheduling of a UL-MU transmission may beindicated through a predetermined field of a preamble of a packet and/ora predetermined field of a MAC header. For example, a STA may indicateinformation for UL-MU transmission scheduling through a predeterminedfield of a preamble or a MAC header of an uplink transmission packet,and may transmit the information to an AP. In this case, the informationfor UL-MU transmission scheduling includes at least one of buffer statusinformation of each STA, channel state information measured by each STA.The buffer status information of the STA may indicate at least one ofwhether the STA has uplink data to be transmitted, the access category(AC) of the uplink data and the size (or the transmission time) of theuplink data.

According to an embodiment of the present invention, the UL-MUtransmission process may be managed by the AP. The UL-MU transmissionmay be performed in response to a trigger frame transmitted by the AP.The STAs simultaneously transmit uplink data a predetermined IFS timeafter receiving the trigger frame. The trigger frame indicates the datatransmission time point of the uplink STAs and may inform the channel(or subchannel) information allocated to the uplink STAs. When the APtransmits the trigger frame, a plurality of STAs transmit uplink datathrough the respective allocated subcarriers at a time point designatedby the trigger frame. After the uplink data transmission is completed,the AP transmits an ACK to the STAs that have successfully transmittedthe uplink data. In this case, the AP may transmit a predeterminedmulti-STA block ACK (M-BA) as an ACK for a plurality of STAs.

In the non-legacy wireless LAN system, a specific number, for example,26, 52, or 106 tones may be used as a resource unit (RU) for asubchannel-based access in a channel of 20 MHz band. Accordingly, thetrigger frame may indicate identification information of each STAparticipating in the UL-MU transmission and information of the allocatedresource unit. The identification information of the STA includes atleast one of an association ID (AID), a partial AID, and a MAC addressof the STA. Further, the information of the resource unit includes thesize and placement information of the resource unit.

On the other hand, in the non-legacy wireless LAN system, a UL-MUtransmission may be performed based on a contention of a plurality ofSTAs for a particular resource unit. For example, if an AID field valuefor a particular resource unit is set to a specific value (e.g., 0) thatis not assigned to STAs, a plurality of STAs may attempt random access(RA) for the corresponding resource unit.

<Retransmission of Trigger Frame>

Hereinafter, embodiments in which a trigger frame is retransmitted inthe uplink multi-user transmission process will be described withreference to the respective drawings. Specifically, a scheduling methodin the case where the AP does not receive any uplink data in response tothe trigger frame will be described.

For efficient scheduling of the UL-MU transmission, various parametersto be used in a series of transmission processes should be determined.For example, the size of the contention window used in the backoffprocedure for transmitting the trigger frame should be determined. Also,a criterion for determining whether or not the transmission of theuplink multi-user data is successful should be established. In addition,the succeeding operation and the backoff method according to the successor failure determination should be defined.

According to the embodiment of the present invention, the AP mayconsider that the transmission process is successful even when a part ofdata is successfully transmitted in the transmission of the uplinkmulti-user data. That is, when uplink data is received from at least oneof the STAs indicated by the trigger frame, the AP determines that theUL-MU transmission process is successful. Thus, the AP transmits an M-BAin response to receiving the uplink multi-user data. On the other hand,when no uplink data in response to the trigger frame is received, the APdetermines that the UL-MU transmission process has failed. According toan embodiment, when the UL-MU transmission process is determined to havefailed, the trigger frame needs to be retransmitted within a time thatthe existing scheduling is valid.

FIGS. 8 to 18 illustrate specific embodiments in which a trigger frameis retransmitted in an uplink multi-user transmission process. In eachof the embodiments of FIGS. 8 to 18 , duplicated descriptions of partswhich are the same or corresponding to those of the embodiments of theprevious drawings will be omitted.

First, referring to FIG. 8 , the AP transmits a trigger frame 310 forinitiating an UL-MU transmission process. The AP may perform a separatebackoff procedure for transmitting the trigger frame 310. When thebackoff procedure for transmitting the trigger frame 310 expires in acontention window interval 42, the AP transmits the trigger frame 310.The trigger frame 310 indicates identification information of each STAparticipating in the UL-MU transmission and information of resourceunits allocated thereto. In the embodiment of FIG. 8 , the trigger frame310 solicits uplink multi-user data transmission of STA1, STA2 and STA3.However, the AP does not receive any uplink data in response to thetrigger frame 310, and performs scheduling for the failure of the UL-MUtransmission process.

According to the embodiment of the present invention, the AP may performretransmission of the trigger frame 310 when the UL-MU transmissionprocess has failed. In this case, the retransmission of the triggerframe 310 may be performed through a channel contention based onexisting DCF rules. For the retransmission of the trigger frame 310, theAP obtains a new backoff counter. In this case, the new backoff countercan be determined within a range of twice the contention window used inobtaining the previous backoff counter. That is, when the UL-MUtransmission process has failed, the AP doubles the size of thecontention window to be used in the backoff procedure of the nextcontention window intervals 44 and 46. In the contention windowintervals 44 and 46, the AP performs a backoff procedure to retransmitthe trigger frame 310 based on the new backoff counter. Theretransmission of the trigger frame 310 may be performed until the UL-MUtransmission based on the retransmission is successful within the presetretransmission limit.

However, since the scheduling applied to multiple users is alreadycompleted when the trigger frame 310 is initially transmitted, there isa high probability that a scheduling error occurs when the trigger frame310 is retransmitted a long time after the initial transmission time.For example, if at least some STAs have already transmitted uplink databefore the retransmission time of the trigger frame 310, the existingscheduling information becomes invalid. Referring to the embodiment ofFIG. 8 , STA2 has won the contention in the next contention windowinterval 44. Accordingly, the STA2 transmits uplink data 340 to the AP,and the AP transmits an ACK 346 in response thereto. In the nextcontention window interval 46, the AP wins the contention andretransmits the trigger frame 310. The STAs receive the trigger frame310 transmitted by the AP and transmit uplink multi-user data 320, thatis, an uplink multi-user PLCP protocol data unit (UL MU PPPU). However,since STA2 indicated by the retransmitted trigger frame 310 has alreadycompleted the transmission of the uplink data 340, there is a problemthat the scheduling information of the retransmitted trigger frame 310is invalid.

Meanwhile, in the embodiment of the present invention, the uplinkmulti-user data 320 may be transmitted in a form including at least oneof OFDMA and MU-MIMO. If the transmission of the uplink multi-user data320 is successful, the AP transmits an M-BA 330 in response thereto. TheM-BA 330 includes ACK information for STAs that have successfullytransmitted the uplink multi-user data 320. In the embodiment of FIG. 8, STA1 and STA3 have successfully transmitted uplink data in response tothe trigger frame 310, and the AP transmits ACK information for STA1 andSTA3 via the M-BA 330.

FIGS. 9 and 10 illustrate a trigger frame retransmission methodaccording to an embodiment of the present invention. According to anembodiment of the present invention, the retransmission timing of thetrigger frame 310 is determined based on the confirmation timing ofwhether or not the uplink multi-user data 322 is received in response tothe trigger frame 310.

Whether or not the uplink multi-user data 322 is received can beconfirmed through the preamble of the UL MU PPDU. If no UL MU PPDUpreamble is received for a predetermined transmission period d_P, the APretransmits the trigger frame 310 immediately when the channel is idle.The retransmission of the trigger frame 310 may be performed until theUL-MU transmission based on the retransmission is successful within apredetermined count limit of retransmission or a predetermined timelimit of retransmission. According to an embodiment, the predeterminedcount limit of retransmission or the predetermined time limit ofretransmission may be determined based on duration of the UL MU PPDUspecified in the trigger frame 310 or a certain ratio thereof.

Referring to the FIG. 9 , when the UL-MU transmission process hasfailed, the AP may attempt retransmission of the trigger frame 310during the first transmission opportunity (TXOP), i.e., TXOP_O set bythe initial trigger frame 310. If the UL MU PPDU preamble is notreceived for a predetermined transmission period d_P and the channel isidle, the AP retransmits the trigger frame 310 after a SIFS time.Accordingly, the AP may retransmit the trigger frame 310 after a time ofSIFS+d_P+SIFS from the initial transmission completion of the triggerframe 310.

When the trigger frame 310 is retransmitted, the STAs may perform theUL-MU transmission during the extended TXOP, i.e. TXOP_E, newlydesignated by the retransmitted trigger frame 310. The STAs receive theretransmitted trigger frame 310 and transmit uplink multi-user data 322in response thereto. In this case, the duration of the uplink multi-userdata 322 is set based on TXOP_E. According to an embodiment, the TXOP_Evalue may be set equal to TXOP_O. Thus, when the trigger frame 310 isretransmitted, the TXOP of the UL-MU transmission may be shifted. In theembodiment of FIG. 9 , STA1, STA2 and STA3 have successfully transmitteduplink data in response to the retransmitted trigger frame 310, and theAP transmits ACK information for STA1, STA2 and STA3 via the M-BA 332.

Meanwhile, according to the embodiment of FIG. 10 , the AP may performthe UL-MU transmission process based on the retransmitted trigger frame310 in a limited manner within the TXOP_O period. That is, when theUL-MU transmission process has failed, the retransmitted trigger frame310 indicates a new TXOP based on the time obtained by subtracting(SIFS+d_P+SIFS) from TXOP_O. Thus, the new TXOP indicated by theretransmitted trigger frame 310 is reduced compared to the O_TXOPindicated by the initial trigger frame 310. The STAs configure uplinkdata based on the reduced TXOP and transmit the configured uplink datato the AP.

As described above, according to the embodiment of FIGS. 9 and 10 , whenthe UL-MU transmission process has failed, the AP retransmits thetrigger frame 310 without any further contention. Accordingly, the APmay increase the probability of success of the UL-MU within the timethat the scheduling information is valid.

FIGS. 11 and 12 illustrated a scheduling method according to consecutivefailure of an uplink multi-user transmission process. As describedabove, the retransmission of the trigger frame 310 may be performeduntil the UL-MU transmission based on the retransmission is successfulwithin the predetermined count limit of retransmission or thepredetermined time limit d_R of retransmission. However, if all theUL-MU transmission processes within the predetermined count limit ofretransmission or the predetermined time limit d_R of retransmissionhave failed, operations of the terminals should be scheduledaccordingly.

If all the UL-MU transmission processes within a predetermined number oftimes or a predetermined period of time have failed, the AP deletes thetrigger frame 310 from the buffer and attempts to transmit another framein the access category queue. The AP may attempt either a downlinksingle-user transmission, a downlink multi-user transmission, or atransmission of a new trigger frame in the next contention windowintervals 44 and 46.

According to the embodiment of FIG. 11 , if the UL-MU transmissionprocess within a predetermined number of times or a predetermined periodof time has failed, the AP may attempt the next transmission withoutincreasing the size of the contention window. That is, the AP performs abackoff procedure based on an initial contention window value accordingto the access category of the packet to be transmitted. The AP obtains anew backoff counter within the initial contention window and accessesthe channel based on the obtained backoff counter.

Referring to the embodiment of FIG. 11 , the AP attempts to transmitdownlink data 350 to STA4 after all the UL-MU transmission processeswithin the predetermined number of times or the predetermined period oftime have failed. The AP obtains a backoff counter using an initialcontention window based on the access category of the downlink data 350and performs a backoff procedure in the contention window intervals 44and 46 based on the obtained backoff counter. In the next contentionwindow interval 44, STA5 wins the contention and transmits uplink data341 to the AP. The AP receives the uplink data 341 transmitted by theSTA5 and transmits an ACK 347 in response thereto. In the nextcontention window interval 46, the AP wins the contention and transmitsdownlink data 350 to the STA4. The STA4 receives the downlink data 350transmitted by the AP and transmits an ACK 356 in response thereto.

Next, according to the embodiment of FIG. 12 , if the UL-MU transmissionprocess within the predetermined number of times or the predeterminedperiod of time has failed, the AP may increase the size of thecontention window for the next transmission. The AP determines that thecontention is very severe when consecutive attempts of the UL-MUtransmission have failed, and increases the size of the contentionwindow to avoid channel congestion.

According to the embodiment of the present invention, the AP increasesthe size of the contention window without performing the retransmissionof the trigger frame 314 when the UL-MU transmission process has failed.When the UL-MU transmission process has failed, the AP may attempt anyone of a downlink single-user transmission, a downlink multi-usertransmission, and a transmission of a new trigger frame in the nextcontention window interval. In this case, the AP may increase thecontention window based on the access category of the packet to betransmitted. That is, when attempting a downlink single-usertransmission or a downlink multi-user transmission, the AP obtains a newbackoff counter by doubling the size of the contention window based onthe access category of data to be transmitted. When transmitting a newtrigger frame, the AP obtains a new backoff counter by doubling the sizeof the existing contention window based on the access category for thetrigger frame. The AP uses the new backoff counter determined based onthe increased contention window to contend with the STAs and access thechannel.

Referring to the embodiment of FIG. 12 , the AP increases the contentionwindow based on the access category of the downlink data 350 and obtainsa backoff counter using the increased contention window. The AP performsa backoff procedure for transmitting the downlink data 350 in thecontention window intervals 44 and 46 based on the obtained backoffcounter.

FIGS. 13 and 14 illustrate an embodiment in which a new trigger frame istransmitted after a failure of the uplink multi-user transmissionprocess. If all of the UL-MU transmission processes within thepredetermined number of times or the predetermined the time period havefailed as described above, the AP may attempt either a downlinksingle-user transmission, a downlink multi-user transmission, or atransmission of a new trigger frame in the next contention windowintervals 44 and 46. In the embodiment of FIGS. 13 and 14 , the APattempts to transmit a new trigger frame 312 in the next contentionwindow intervals 44 and 46.

According to the embodiment of FIG. 13 , the AP may attempt to transmita new trigger frame 312 without increasing the size of the contentionwindow. That is, the AP obtains a new backoff counter within the initialcontention window based on the access category for the trigger frame.The AP contends with the STAs based on the new backoff counter andaccesses the channel. The AP may obtain a new backoff counter withoutincreasing the size of the contention window even though the previousUL-MU transmission process has finally failed.

Referring to the embodiment of FIG. 13 , the AP obtains a backoffcounter using the initial contention window based on the access categoryof the trigger frame 312, and performs a backoff procedure in thecontention window intervals 44 and 46 based on the obtained backoffcounter. In the next contention window interval 44, STA2 wins thecontention and transmits uplink data 340 to the AP. The AP receives theuplink data 340 transmitted by the STA2 and transmits an ACK 346 inresponse thereto. In the next contention window interval 46, the AP winsthe contention and transmits a new trigger frame 312. The new triggerframe 312 solicits uplink multi-user data transmission of STA1, STA3 andSTA4. The STAs receive the new trigger frame 312 transmitted by the APand transmit uplink multi-user data 324 in response thereto. In theembodiment of FIG. 13 , STA1, STA3 and STA4 have successfullytransmitted uplink data in response to the new trigger frame 312, andthe AP transmits ACK information for STA1, STA3 and STA4 via the M-BA334.

Next, according to the embodiment of FIG. 14 , the AP may attempt totransmit a new trigger frame 312 after increasing the size of thecontention window. The AP increases the contention window based on theaccess category for the trigger frame. According to an embodiment, theAP may double the size of the existing contention window based on theaccess category for the trigger frame. The AP obtains a new backoffcounter based on the increased contention window and performs a backoffprocedure for transmitting the new trigger frame 312 in the contentionwindow intervals 44 and 46 based on the obtained new backoff counter.

FIGS. 15 and 16 illustrate a trigger frame retransmission methodaccording to another embodiment of the present invention. According toanother embodiment of the present invention, the AP may performretransmission of the trigger frame based on the PIFS recovery scheme.In this case, the AP may apply the PIFS recovery scheme to thetransmission of the first trigger frame for securing a TXOP.

In the conventional PIFS recovery scheme, when the first frame of a setTXOP is successfully transmitted, the subsequent frames in the same TXOPcan be retransmitted after a PIFS time in case of transmission failure.That is, when the transmission of the subsequent frame has failed, thecorresponding frame is retransmitted after a PIFS time without anycontention. According to an embodiment of the present invention, the APapplies such PIFS recovery scheme to the retransmission of the triggerframe. That is, when the first frame transmitted to secure a TXOP is atrigger frame (i.e., the first trigger frame), the PIFS recovery schemeis also applied to the corresponding trigger frame. The AP transmits thefirst trigger frame for initiating the UL-MU transmission process andtransmits the second trigger frame when the channel is idle for a PIFStime from the transmission of the first trigger frame. In this case, thetransmitted second trigger frame may be the retransmitted first triggerframe, or may be a new trigger frame.

Referring to the embodiment of FIG. 15 , the AP immediately performsretransmission of the trigger frame 310 when the channel is idle for aPIFS time after the transmission of the first trigger frame 310 of theUL-MU transmission process. According to an embodiment, such aretransmission of the trigger frame 310 may be performed until theretransmission is successful within the first TXOP, i.e., TXOP_O set inthe initial trigger frame 310.

When the trigger frame 310 is retransmitted, the STAs may perform theUL-MU transmission during an extended TXOP, i.e. TXOP_E, newlydesignated by the retransmitted trigger frame 310. In the embodiment ofFIG. 15 , STAs receive the retransmitted trigger frame 310 and transmituplink multi-user data 322 in response thereto. In this case, theduration of the uplink multi-user data 322 is set based on TXOP_E.According to an embodiment, the TXOP_E value may be set equal to TXOP_O.Thus, when the trigger frame 310 is retransmitted, the TXOP of the UL-MUtransmission may be shifted. In the embodiment of FIG. 15 , STA1, STA2and STA3 have successfully transmitted uplink data in response to theretransmitted trigger frame 310, and the AP transmits ACK informationfor STA1, STA2 and STA3 via the M-BA 332.

Referring to the embodiment of FIG. 16 , the AP may transmit a newtrigger frame 314 when the channel is idle for a PIFS time after thetransmission of the first trigger frame 310 of the UL-MU transmissionprocess. The new trigger frame 314 may have different schedulinginformation than the previously transmitted trigger frame 310. In theembodiment of FIG. 16 , the initially transmitted trigger frame 310solicits the uplink multi-user data transmission of STA1, STA2 and STA3,but the retransmitted trigger frame 314 solicits the uplink multi-userdata transmission of STA4, STA5 and STA6 based on new schedulinginformation. STA4, STA5, and STA6 transmit uplink multi-user data 326 inresponse to the retransmitted trigger frame 314, and the AP receives theuplink multi-user data 326. The AP transmits ACK information of STA4,STA5 and STA6 via the M-BA 336 in response to the received uplinkmulti-user data 326.

As described above, according to the embodiment of FIGS. 15 and 16 , ifthe UL-MU transmission process has failed, the AP retransmits thetrigger frame after a PIFS time without any contention.

FIGS. 17 and 18 illustrate a trigger frame retransmission methodaccording to yet another embodiment of the present invention. Accordingto yet another embodiment of the present invention, the AP may performthe retransmission of the trigger frame based on the aforementioned PIFSrecovery scheme for a predetermined retransmission time limit d_R.

First, referring to the embodiment of FIG. 17 , if the UL-MUtransmission process corresponding to the transmission of the triggerframe 310 has failed, the AP performs retransmission of the triggerframe 310 based on the PIFS recovery within the d_R time. In this case,the AP may perform the retransmission of the trigger frame 310 based onthe PIFS recovery even if the channel is not idle immediately after thetransmission of the initial trigger frame 310. For example, there may bea case where interference occurs between neighboring BSSs, collisionoccurs between frames, or a received frame is not decodable. In thiscase, the AP performs retransmission of the trigger frame 310 after aPIFS time from the time when the channel becomes idle. Meanwhile, FIG.17 illustrates an embodiment in which the same trigger frame 310 isretransmitted based on the PIFS recovery, but the present invention isnot limited thereto. That is, the AP may transmit a new trigger framebased on new scheduling information within d_R time.

If all the UL-MU transmission processes within the predeterminedretransmission time limit d_R have failed, the AP terminates thetransmission of the corresponding trigger frame 310 and attempts totransmit another frame in the access category queue. The AP may attempteither a downlink single-user transmission, a downlink multi-usertransmission, or a transmission of a new trigger frame in the nextcontention window intervals 44 and 46. According to the embodiment ofFIG. 17 , if the UL-MU transmission process within the d_R time periodhas failed, the AP may attempt the next transmission without increasingthe size of the contention window. That is, the AP performs the backoffprocedure based on the initial contention window value according to theaccess category of the packet to be transmitted. The AP obtains a newbackoff counter within the initial contention window and accesses thechannel based on the obtained backoff counter.

Referring to the embodiment of FIG. 17 , the AP attempts to transmit anew trigger frame 312 after all the UL-MU transmission processes withind_R time have failed. The AP obtains a backoff counter using the initialcontention window based on the access category of the trigger frame 312and performs a backoff procedure in the contention window intervals 44and 46 based on the obtained backoff counter. In the next contentionwindow interval 44, STA2 wins the contention and transmits uplink data340 to the AP. The AP receives the uplink data 340 transmitted by theSTA2 and transmits an ACK 346 in response thereto. In the nextcontention window interval 46, the AP wins the contention and transmitsa new trigger frame 312. The new trigger frame 312 solicits uplinkmulti-user data transmission of STA1, STA3 and STA4. The STAs receivethe new trigger frame 312 transmitted by the AP and transmit uplinkmulti-user data 324 in response thereto. In the embodiment of FIG. 17 ,STA1, STA3 and STA4 have successfully transmitted uplink data inresponse to the new trigger frame 312, and the AP transmits ACKinformation for STA1, STA3 and STA4 via the M-BA 334.

Next, according to the embodiment of FIG. 18 , if all the UL-MUtransmission processes within the d_R time have failed, the AP mayincrease the size of the contention window for the next transmission.The AP determines that the contention is very severe when the UL-MUtransmission attempt based on the PIFS recovery have failed, andincreases the size of the contention window to avoid channel congestion.The AP obtains a new backoff counter within the increased contentionwindow and accesses the channel. A detailed exemplary embodiment thereofis as described above in the embodiment of FIG. 12 .

<Cascaded Uplink Multi-User Transmission>

FIGS. 19 to 21 illustrate consecutive uplink multi-user transmissionmethods according to an embodiment of the present invention. Uplinkmulti-user data transmission is a scheme that maximizes spectralefficiency in terms of reducing overhead due to backoff contention amonga plurality of STAs. Therefore, when starting the first uplinkmulti-user data transmission session, the AP may increase thetransmission efficiency by setting a NAV to a length of the maximumavailable TXOP and triggering uplink multi-user data transmission asshown in FIG. 15 . That is, the duration field of the trigger frame(i.e., TF) transmitted first for the uplink multi-user data transmissionmay be set to a value corresponding to the maximum TXOP. The AP obtainsbuffer state information of STAs in the BSS in advance, and allocatesresources for uplink data transmission in consideration of uplink datasize information, uplink data AC information, and channel selectivity ofeach STA. In the following embodiments and drawings, TF denotes atrigger frame, UL-DATA denotes uplink multi-user data, and M-BA denotesmulti-STA block ACK.

FIG. 19 illustrates a consecutive uplink multi-user transmission methodaccording to an embodiment of the present invention. First, referring toFIG. 19 (a), transmissions of a trigger frame 410, uplink multi-userdata 420, and an M-BA 430 according to the aforementioned embodiment maybe performed. If a NAV of a sufficient length remains for additionalUL-MU transmission even after the completion of the transmission of theM-BA 430, the AP triggers the next UL-MU transmission without returningthe NAV. To this end, the AP transmits a new trigger frame 412 SIFSafter the M-BA 430 has been transmitted. A new UL-MU transmissionprocess is performed by the transmission of a new trigger frame 412.That is, transmissions of uplink multi-user data 422 in response to thenew trigger frame 412 and an M-BA 432 are additionally performed withinthe maximum TXOP.

According to an embodiment of the present invention, the M-BA 430 andthe new trigger frame 412 may be combined and transmitted in a singleaggregated MAC protocol data unit (A-MPDU). That is, when the uplinkmulti-user data 420 in response to the first trigger frame 410 isreceived, the AP may transmit an A-MPDU combined with the M-BAcorresponding to the uplink multi-user data 420 and a second triggerframe. Accordingly, the ACK transmission for the previous UL-MUtransmission and the trigger for the next UL-MU transmission can beperformed through one frame.

FIG. 19 (b) illustrates a situation in which a NAV does not remain for asufficient length for additional UL-MU transmission after the completionof transmission of the M-BA. According to an embodiment, the AP maytransmit a CF-END frame 440 to terminate the UL-MU transmission processand return the NAV. According to another embodiment of the presentinvention, the AP may perform transmission of a downlink control frame,a management frame, etc. of a short length during the remaining NAVinterval.

As described above, according to the embodiment of the presentinvention, consecutive UL-MU transmissions may be performed for apredetermined period. According to an embodiment, the predeterminedperiod may be a predefined maximum TXOP. According to anotherembodiment, the AP may flexibly select the predetermined period inconsideration of the number of terminals in the BSS, the uplink/downlinktransmission ratio, the traffic congestion, and the like.

FIG. 20 illustrates a cancellation method of a consecutive uplinkmulti-user transmission process according to another embodiment of thepresent invention. When the network is congested, the STAs may not beable to transmit uplink data in response to the trigger frame 410 of theAP, or the AP may not be able to receive uplink data of the STAs.Alternatively, the AP may receive only a part of uplink data 424 inresponse to the trigger frame 410. In this case, the subsequent UL-MUtransmission process may also be predicted to be difficult due to thecongestion situation.

Therefore, as in the embodiment of FIG. 20 (a), if uplink multi-userdata in response to the trigger frame 410 is not received, the AP maytransmit the CF-END frame 440 to cancel the TXOP and terminate the UL-MUtransmission process. On the other hand, as in the embodiment of FIG. 20(b), when a part of uplink multi-user data 424 in response to thetrigger frame 410 is received, the AP may transmit an M-BA 434corresponding to the data 424 and transmit the CF-END frame 440thereafter. According to an embodiment, the AP may transmit the CF-ENDframe 440 when uplink multi-user data 424 is received from less than orequal to a predetermined number of STAs in response to the trigger frame410.

FIG. 21 illustrates a consecutive uplink multi-user transmission methodusing multiple channels according to yet another embodiment of thepresent invention. According to the embodiment of the present invention,the aforementioned consecutive uplink multi-user transmission processmay be performed through a wideband channel of 20 MHz or more. FIG. 21shows an embodiment in which an uplink multi-user transmission processis performed through a primary channel, a secondary channel 1 and asecondary channel 2. In the embodiment of FIG. 21 , duplicateddescriptions of parts which are the same or corresponding to those ofthe aforementioned embodiments of FIGS. 19 and 20 will be omitted.

First, the AP transmits trigger frames 410 a, 410 b and 410 c through aplurality of channels, that is, a primary channel, a secondary channel1, and a secondary channel 2. The trigger frames 410 a, 410 b and 410 cmay be transmitted via separate frames on a 20 MHz channel basis.Alternatively, the trigger frames 410 a, 410 b and 410 c may betransmitted via one frame through a plurality of channels. In theembodiment of FIG. 21 , the trigger frame 410 a transmitted through theprimary channel solicits uplink multi-user transmission by allocatingthe primary channel to STAs having AID values of 63, 457, 91 and 78. Thetrigger frame 410 b transmitted through the secondary channel 1 solicitsuplink multi-user transmission by allocating the secondary channel 1 toSTAs having AID values of 1, 3, 12 and 45. In addition, the triggerframe 410 c transmitted through the secondary channel 2 solicits uplinkmulti-user transmission by allocating the secondary channel 2 to STAshaving AID values of 37, 107, 6 and 54.

However, the communicable channel of each terminal in the BSS may bedifferent depending on the position of the corresponding terminal andthe channel occupancy status of the neighboring BSSs. Accordingly, asituation may occur in which a resource unit is allocated through thetrigger frame transmitted by the AP but the STA fails to transmit uplinkmulti-user data. According to the embodiment of the present invention,when uplink multi-user data is not received from a STA designated by thetrigger frame, the AP may allocate another channel (or resource unit) tothe corresponding STA in the next UL-MU transmission process to solicituplink multi-user transmission. That is, when uplink data is notreceived from a STA to which the first channel (or the first resourceunit) is allocated through the first trigger frame, the AP allocates thesecond channel (or the second resource unit) to the STA in the nextsecond trigger frame to solicit uplink multi-user transmission. In thiscase, the first trigger frame and the second trigger frame are differenttrigger frames transmitted within a predetermined period (for example,maximum TXOP) for consecutive UL-MU transmissions.

Referring to the embodiment of FIG. 21 , a second UL-MU transmissionprocess based on a second trigger frame (412 a, 412 b and 412 c) isinitiated by following a first UL-MU transmission process based on afirst trigger frame (410 a, 410 b and 410 c). The AP initiates thesecond UL-MU transmission process by allocating another channel to a STAthat has failed to transmit uplink multi-user data in the first UL-MUtransmission process. For example, a STA of AID 78 allocated with theprimary channel by the first trigger frame 410 a is newly allocated withthe secondary channel 1 by the second trigger frame 412 b. In addition,STAs of AID 12 and AID 45 allocated with the secondary channel 1 by thefirst trigger frame 410 b are newly allocated with the primary channeland the secondary channel 2 by the second trigger frames 412 a and 412c, respectively.

As described above, in the consecutive UL-MU transmission process, theAP generates new scheduling information for STAs that have failed totransmit uplink multi-user data, and transmits a trigger frame based onthe new scheduling information.

FIG. 22 illustrates a method of a buffer status report and an uplinkmulti-user transmission based thereon according to an embodiment of thepresent invention. First, referring to FIG. 22 (a), in order to increasethe spectrum efficiency of the UL-MU transmission process, a bufferstatus report (BSR, 452) of STAs may be transmitted before the UL-MUtransmission starts. In order to solicit the STAs to transmit bufferstatus report 452, the AP may transmit a buffer status report request(BSRR, 450). STAs having uplink data to be transmitted among the STAshaving received the buffer status report request 450 randomly select oneof the resource units allocated for the buffer status report 452 andtransmits the buffer status report 452. Upon receiving the buffer statusreport 452, the AP allocates a resource unit based on the secured bufferstatus information and transmits a trigger frame 410 for startingtransmission of the UL-DATA.

FIG. 22 (b) illustrates a configuration of a buffer status reportaccording to an embodiment of the present invention. According to theembodiment of the present invention, the buffer status reporttransmitted by the STA includes at least one of identifier informationof the corresponding STA, size (i.e., length) information of data to betransmitted by the STA, and an access category (AC) of data to betransmitted by the STA. The identifier information of the STA representsone of an AID, a partial AID, or a MAC address of the STA.

In the embodiment of FIG. 22 (b), the AP has allocated resource unitsfor transmitting buffer status reports to a total of four channels inunits of 20 MHz, that is, a primary channel, a secondary channel 1, asecondary channel 2, and a secondary channel 3. In this case, theresource units for transmitting the buffer status report may beallocated in units of sub-channels further subdivided in the 20 MHzchannel. In the embodiment of FIG. 22 (b), each allocated resource unitrepresents a random access-based resource unit. STAs having uplink datato be transmitted select any resource unit among the allocated resourceunits and transmit a buffer status report composed of the aboveinformation.

According to an embodiment of the present invention, an additionalcontrol method for reducing the probability of a collision occurrence ofa random access based buffer status report transmission in a networkcongestion state can be used. The STA may obtain a transmissionprobability based on the access category to be transmitted by thecorresponding terminal, and may transmit the buffer status report basedon the obtained transmission probability. Table 1 shows transmissionprobabilities of buffer status report according to the access categoriesof data.

TABLE 1 Management P0 AC_VO P1 AC_VI P2 AC_BE P3 AC_BK P4

Here, P0>P1>P2>P3>P4.

In addition, AC_VO indicates a voice access category, AC_VI indicates avideo access category, AC_BE indicates a best effort access category,and AC_BK indicates a background access category.

According to the embodiment of the present invention, STAs transmittingthe buffer status report allocate different probability values accordingto the access category of data to be transmitted to perform adifferential access. That is, the STA having data of an access categorywith a high probability value has the higher transmission probability ofthe buffer status report. According to an embodiment, the higher thepriority of the access category of data to be transmitted is, the higherthe transmission probability of the buffer status report is. Uponreceiving the buffer status report request, the STAs determine whetheror not to transmit the buffer status report based on the probabilitydetermined according to the access category of the data in the buffer ofthe corresponding terminal.

FIGS. 23 and 24 illustrate an uplink multi-user transmission processaccording to a further embodiment of the present invention. When thetrigger frame is transmitted, STAs should transmit uplink multi-userdata after a predetermined IFS time. In this case, the STAs needprocessing time for receiving the trigger frame and participating in theUL-MU transmission in response thereto.

First, according to the embodiment of FIG. 23 , padding can be used tosecure the processing time for the UL-MU transmission. First, the APtransmits a downlink packet 510 including a trigger frame for startingthe UL-MU transmission. According to an embodiment, the AP may transmita downlink packet 510 in which downlink data and the trigger frame areaggregated. In the embodiment of FIG. 23 , M-TF denotes a trigger frameindicating multiple users, and S-TF denotes a trigger frame indicating asingle user. According to an embodiment, the AP may transmit triggerinformation for a receiving STA of the downlink data included in thedownlink packet 510 via the S-TF and trigger information for other STAsvia the M-TF.

When the downlink packet 510 including the trigger frame is transmitted,the STAs transmit an uplink packet 520 including uplink multi-user datain response thereto. According to an embodiment, the uplink packet 520in response to the trigger frame may be transmitted a SIFS time afterthe transmission of the trigger frame has been completed. Accordingly,the uplink packet 520 may be transmitted after the downlink packet 510of the AP is transmitted and before an ACK timer of the AP expires. Inthe embodiment of FIG. 23 , uplink data of STA1, STA3, STA4 and STA5 istransmitted along with a block ACK of STA2 via the uplink packet 520.

According to an embodiment of the present invention, a PHY padding 522may be performed on the uplink packet 520 transmitted in response to thetrigger frame. That is, STAs to transmit the uplink multi-user datatransmit the preamble of the uplink packet 520 a SIFS time after thereception of the trigger frame, and transmit a padding 522 waveformuntil the transmission of the uplink data (e.g., A-MPDU) becomesavailable. Assuming that the time from when the trigger frame isreceived to when the uplink data (e.g., A-MPDU) is transmitted is anxIFS, the STAs transmitting the uplink multi-user data perform the PHYpadding 522 for a time of xIFS−SIFS. In this case, the xIFS has a valuelarger than the SIFS. The STAs transmit the uplink multi-user dataimmediately after the PHY padding 522. The AP receives the uplinkmulti-user data transmitted by the STAs and transmits an M-BA 530 inresponse thereto.

Next, according to the embodiment of FIG. 24 , an extended ACK timer maybe applied to secure the processing time for the UL-MU transmission.That is, the uplink packet 520 in response to the trigger frame may betransmitted a predetermined xIFS time after the transmission of thetrigger frame has been completed. In this case, the predetermined xIFSis set to the time of a SIFS+n*slot (where n is an integer of 1 ormore). In the embodiment of FIG. 24 , since the extended ACK timer isapplied, the STAs can secure the time for configuring the uplinkmulti-user data in response to the trigger frame. Accordingly, the STAsmay transmit the uplink data (e.g., A-MPDU) immediately after thepreamble of the uplink packet 520.

Although the present invention is described by using the wireless LANcommunication as an example, the present invention is not limitedthereto and the present invention may be similarly applied even to othercommunication systems such as cellular communication, and the like.Further, the method, the apparatus, and the system of the presentinvention are described in association with the specific embodiments,but some or all of the components and operations of the presentinvention may be implemented by using a computer system having universalhardware architecture.

The detailed described embodiments of the present invention may beimplemented by various means. For example, the embodiments of thepresent invention may be implemented by a hardware, a firmware, asoftware, or a combination thereof.

In case of the hardware implementation, the method according to theembodiments of the present invention may be implemented by one or moreof Application Specific Integrated Circuits (ASICSs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, micro-controllers, micro-processors,and the like.

In case of the firmware implementation or the software implementation,the method according to the embodiments of the present invention may beimplemented by a module, a procedure, a function, or the like whichperforms the operations described above. Software codes may be stored ina memory and operated by a processor. The processor may be equipped withthe memory internally or externally and the memory may exchange datawith the processor by various publicly known means.

The description of the present invention is used for exemplification andthose skilled in the art will be able to understand that the presentinvention can be easily modified to other detailed forms withoutchanging the technical idea or an essential feature thereof. Thus, it isto be appreciated that the embodiments described above are intended tobe illustrative in every sense, and not restrictive. For example, eachcomponent described as a single type may be implemented to bedistributed and similarly, components described to be distributed mayalso be implemented in an associated form.

The scope of the present invention is represented by the claims to bedescribed below rather than the detailed description, and it is to beinterpreted that the meaning and scope of the claims and all the changesor modified forms derived from the equivalents thereof come within thescope of the present invention.

INDUSTRIAL APPLICABILITY

Various exemplary embodiments of the present invention have beendescribed with reference to an IEEE 802.11 system, but the presentinvention is not limited thereto and the present invention can beapplied to various types of mobile communication apparatus, mobilecommunication system, and the like.

The invention claimed is:
 1. A station (STA) comprising: a transceiverconfigured to transmit and receive wireless signals; and a processorconfigured to process wireless signals transmitted or received throughthe transceiver, wherein the processor is configured to: receive a firsttrigger frame indicating transmission of an uplink data to one or moreSTAs for a multi-user transmission through a channel from an accesspoint (AP), wherein the first trigger frame includes resource allocationinformation for the one or more STAs, transmit the uplink data inresponse to the first trigger frame based on the resource allocationinformation, and receive a second trigger frame after a pointcoordination function (PCF) interframe space (PIFS) time from the APwhen a specific condition is satisfied, wherein the specific conditionindicates at least one condition for transmitting the second triggerframe after the first trigger frame is transmitted, wherein the at leastone condition includes that the channel is determined to be idle duringthe PIFS time, and the AP does not receive at least one uplink data inresponse to the first trigger frame from the one or more STAs, andwherein the channel is determined to be idle during the PIFS time fortransmission of the second trigger frame through a carrier sensingprocedure.
 2. The STA of claim 1, wherein the at least one conditionfurther includes that a predetermined transmission opportunity (TXOP) issufficient for transmission of the second trigger frame.
 3. The STA ofclaim 1, wherein the multi-user transmission for the at least one uplinkdata is determined to be successful when the AP receives the at leastone uplink data in response to the first trigger frame from the one ormore STAs.
 4. The STA of claim 1, Wherein the multi-user transmissionfor the at least one uplink data is determined to be failed when the APdoes not receive the at least one uplink data in response to the firsttrigger frame from the one or more STAs.
 5. The STA of claim 4, whereina new back-off counter for transmitting the second trigger frame isgenerated when the AP does not receive the at least one uplink data inresponse to the first trigger frame, and wherein the new backoff counteris used by the AP to perform a backoff procedure.
 6. A wirelesscommunication method of a station (STA), the method comprising:receiving a first trigger frame indicating transmission of an uplinkdata to one or more STAs for a multi-user transmission through a channelfrom an access point (AP), wherein the first trigger frame includesresource allocation information for the one or more STAs; transmittingthe uplink data in response to the first trigger frame based on theresource allocation information; and receiving a second trigger frameafter a point coordination function (PCF) interframe space (PIFS) timefrom the AP when a specific condition is satisfied, wherein the specificcondition indicates at least one condition for transmitting the secondtrigger frame after the first trigger frame is transmitted, wherein theat least one condition includes that the channel is determined to beidle during the PIFS time, and the AP does not receive at least oneuplink data in response to the first trigger frame from the one or moreSTAs, and wherein the channel is determined to be idle during the PIFStime for transmission of the second trigger frame through a carriersensing procedure.
 7. The method of claim 6, wherein the at least onecondition further includes that a predetermined transmission opportunity(TXOP) is sufficient for transmission of the second trigger frame. 8.The method of claim 6, wherein an uplink multi-user transmission for theat least one uplink data is determined to be successful when the APreceives the at least one uplink data in response to the first triggerframe from the one or more STAs.
 9. The method of claim 6, wherein anuplink multi-user transmission for the at least one uplink data isdetermined to be failed when the AP does not receive the at least oneuplink data in response to the first trigger frame from the one or moreSTAs.
 10. The method of claim 9, wherein a new back-off counter fortransmitting the second trigger frame is generated when the AP does notreceive the at least one uplink data in response to the first triggerframe, and wherein the new backoff counter is used by the AP to performa backoff procedure.